Re-constructing historical landscapes

SHA 2017 GIS workshop unit 3

Historical imagery

Raster data

  • Grid of cells that form a continuous surface
  • Each grid cell contains a value that represents some real-world phenomena (e.g. elevation, land use)

Quantitative and qualitative rasters

Raster data resolution

  • Cell dimension: the size of each grid cell (e.g. 30 meters by 30 meters)
  • Resolution: the relative size of the grid cell

Types of raster data

  • ArcGIS and other GIS packages can handle a variety of raster formats, including:
  • Image files (JPEG, TIFF, BMP, PNG, etc.)
  • Geo-enhanced image files (GeoTIFF, GeoJPEG)
  • ESRI GRID, ESRI ASCII GRID
  • Remotely sensed imagery (e.g. ERDAS Imagine .img files)
  • Many more.

3D coordinate systems

  • Z-coordinates: vertical measurements (elevation), or measurement of magnitude
Source: Bolstad, GIS Fundamentals

Vertical datums

  • Reference for measuring heights/elevation
  • Common vertical datums:
  • Mean sea level
  • The geoid (orthometric heights)
Source: Bolstad, GIS Fundamentals

Digital Elevation Models (DEMs)

  • Raster representation of a terrain’s (commonly the Earth’s) surface

How DEMs are produced

  • Original elevation measurements taken by field surveying
  • Additional information supplied by:
  • Analysis of remotely sensed imagery / photogrammetry
  • Laser technology (Light Detection and Ranging, or LiDAR)
  • Data: USGS 3D Elevation Program

DEMs in ArcMap

Surface (terrain) analysis

  • Used to model the topographic characteristics of a given landscape
  • Applications:
  • Shaded relief mapping
  • Construction and transportation planning
  • Hydrologic modeling (flood zones, river flow, watershed boundary delineation)

Slope

  • Definition: how elevation changes with horizontal position (rise over run!)
  • In a GIS, slope is often measured in degrees

How slope is calculated

Aspect

  • The steepest downhill direction at a given point; can be thought of as the direction of the slope

Hillshading

  • Representation of three-dimensional characteristics of a two-dimensional surface
  • Used to produce shaded relief maps

Surface analysis in ArcMap

Contours

  • Lines that connect cells of equal value in a raster (e.g. isolines)
  • Applications: elevation, temperature, pollution, etc.

Viewshed analysis

  • Viewshed: the cells in a raster that can be viewed from a given point(s)
  • Lines of sight calculated for points based on a DEM
Source: Bolstad, GIS Fundamentals

3D GIS concepts

  • Extrusion: vertical stretching of a 2D shape for display in 3D, based on some attribute or value

  • Base heights: how elevation/height values are defined within a three-dimensional GIS view

  • Drape: display of a layer in three dimensions given the defined base heights

3D visualization in ArcScene